Sadly, reality does not agree with mikezs today. Aluminum square taper cranks often get rounded out and loose like d_D says. It is almost always the left crank. But it usually only happens when the cranks are not properly installed. I have seen it happen occaisionally on older bikes, too, though.

THe characteristic 'creak' of a loose crank arm can generally be differentiated from other creaks with the following test:

1. With all your weight on the saddle, push really hard down on the left pedal when it is in front
2. Take your foot off the left pedal
3. Use the right pedal to turn the cranks so that the cranks are level (at 3:00 and 9:00) and so the left pedal is at the back, keeping your weight on the saddle and left foot off the left pedal
4. Gently place your foot back on the left pedal
5. Stand up suddenly and listen and feel.
If your square taper it getting worn or damaged and is squeaking, this procedure will give you the best chance of reproducing the squeak. You will be able to hear it and possibly feel it in your foot.